Process for electrochemically dissolving a metal such as zinc or tin

ABSTRACT

An improved electrochemical dissolution process for electrochemically dissolving a first metal by simultaneously creating hydrogen evolution at a second metal. The second metal is a metal that has a larger current exchange density for hydrogen evolution than the first metal, and both metals are immersed in an aqueous electrolyte system, wherein the first metal and the second metal are galvanically coupled. By taking measures to reduce inhibition of the hydrogen evolution at the second metal, the rate of dissolution of the first metal is enhanced. The measures to reduce the inhibition comprise selecting suitable temperatures and concentrations of the electrolyte, dividing the electrolyte into two fluids coupled by a selectively permeable device and selecting suitable resistance value for a connecting means electrically connecting the first and the second metal. The invention is particularly useful for removing Zn or Sn from Zn- or Sn-containing steel scrap.

TECHNICAL FIELD

The invention relates to a method for electrochemically dissolving afirst metal such as Zn or Sn by simultaneously creating hydrogenevolution at a second metal, and to a use of such a method for treatingscrap and to an improved method of dezincing steel scrap.

BACKGROUND ART

Examples of industrial applications for dissolution of a metal, such asZn or Sn in an aqueous electrolyte are the preparing of a solutioncontaining ions of zinc or tin for electroplating purposes or dezincingor detinning of metal scrap, especially of steel scrap.

For preparing or replenishing a solution for plating purposes a metalmay be fed to the aqueous electrolyte in a substantially pure form andno selective dissolution with respect to other metals may be needed. Forremoving the metal from e.g. steel scrap selective dissolution isdesired, selective in that only the metal(s) to be dissolved dissolve,in order to be able to separate the metal from the scrap thus obtainingthe separate metal and steel scrap substantially free of that metal.

As Sn and Zn may be applied as a covering layer on steel byelectroplating from an electrolyte and the resulting steel is to berecycled, both above-mentioned dissolution applications are particularlyrelevant for steel production and recycling. Tinned steel is a widelyused packaging material and galvanized steel is used in numerous productapplications for example in automotive applications. In anelectrochemical process for selective dissolution of Zn and Sn thesemetals and also Pb and Al may be separated from steel scrap, thusproviding for recovery of such metals as well as clean steel scrap thatcan be reused in the manufacturing of steel.

The need for high efficiency removal of especially Zn from steel scraphas increased lately because the production of galvanized steel hasincreased enormously in the last 20 years, particularly in the buildingindustry and in the automotive industry as well as for donesticappliances. In the life cycle sooner or later such Zn-containing steelwill form scrap which is to be reprocessed in a steel making process forwhich only a limited Zn content of the scrap is allowed or desired.

In a known process for dissolving metals in an aqueous electrolyte forpreparing a solution for plating purposes electrical power is fed to theelectrolytical process in order to obtain an acceptable dissolutionrate. In order to obviate electrodeposition of the metal being dissolvedin the aqueous electrolyte on the cathode, a membrane may be installedseparating the anolyte and the catholyte.

In dezincing or detinning metal scrap which takes place in an alkalineelectrolyte for selectivity, dissolution is e.g. carried out withoutelectrical power supplied making use of intrinsic galvanic couplingbetween different metals in the scrap in which case dissolution ratesare very low, or electrical power is supplied from an external sourcesuch as a rectifier to the process.

U.S. Pat. No. 5,302,260 discloses a method of removing zinc fromganvalized steel without substantial co-dissolution of substrate ironcomprising immersing the ganvalized steel in a caustic electrolytesolution anid electrically connecting the ganvalized steel to a cathodematerial which is stable in the caustic electrolyte and has a lowhydrogen voltage.

EP 0 479 326 discloses alkaline leaching for dezincing of ganvalizedsteel scrap wherein the leaching solution includes an oxidant.

DISCLOSURE OF THE INVENTION

It is the object of the invention to improve the efficiency of thedissolution process by reducing the consumption of energy or materialsand/or obviating the need to install electrical power and apparatusesfor direct current supply.

This object is accomplished by employing the method according to claim1.

In the present invention a galvanic process is used, i.e. a processwherein the first and the second metal are coupled without any externalelectrical power being fed to the process. It is remarked that agalvanic process per se is known, and is known to have the disadvantagethat the dissolution rate almost immediately after immersion falls to avery low level, viz, as soon as the concentration of the first metal inthe aqueous electrolyte begins to rise.

According to the invention in a method using such a galvanic process newand inventive measures can be taken to promote the evolution or hydrogenat the second metal thereby enhancing the dissolution rate of the firstmetal, which measures turn out to be of a quite simple nature.

In a preferred embodiment the aqueous electrolyte is an alkalinesolution. Herewith the advantage is obtained that in the case ofprocessing steel scrap the steel base is passivatedr in other words isnot being dissolved. Also the equipment parts can be made using steel.

It is preferred that the alkaline solution has an alkalinity of morethan 8M, preferably more than 9M. The dissolution rate rapidly increasesfor hydroxide concentrations above said values. This rapid increase isunexpected because in the region of hydroxide concentrations up to 7-8Mthe dissolution rate increases disproportionally with a decreasingslope.

It is preferred that the alkaline solution is held at a temperature ofabove 340 K, preferably above 350 K. Above this temperature thedissolution rate substantially increases.

Further it is preferred that mechanical abrasion is carried out or thesurface of the second metal. This also promotes the hydrogen evolution.

Also the hydrogen evolution is promoted if powder of a second metal isadded to the aqueous electrolyte surrounding the second metal agitatingthe aqueous solution containing the powder.

In an embodiment where the first metal is in the form of separateelements the first and the second metal are coupled by a currentcollector contacting the first metal. The current collector thenelectrically connects the first and the second metal. Such a currentcollector may be a metal casing containing the electrolyte.

An active surface of Mg is advantageous if it is desired that nohydrogen evolution occurs as a consequence of the presence of thecurrent collector, since under these conditions at Mg no hydrogenevolution occurs.

In a very interesting embodiment of the method according to theinvention the first and the second metal are galvanically coupled byconnecting means said connecting means providing an electricalresistance which is selected such that the current flow through saidconnecting means is substantially at the maximum value obtainable byvarying the resistance. Surprisingly an optimum resistance value notnecessarily being the minimum resistance of the connecting means can beselected for maximum current flow through said connecting means, whichmaximum current flow corresponds to the maximum dissolution rateobtainable. If the resistance is lowered from infinity to zero, thecurrent flow through the resistance firstly expectedly rises. However,surprisingly if the resistance is lowered below a specific resistancevalue, the current flow unexpectedly drops. According to the inventionan optimum resistance value can be selected for maximum current flow andthus for maximum dissolution rate.

In a particular embodiment the electrical circuit comprising theconnecting means is periodically interrupted. In cases where theinhibition of hydrogen evolution develops at a lower speed than itsubsides upon interruption of the circuit, by suitably switching on andoff the galvanic process a higher integral yield can be effected intime.

In a most preferred embodiment the aqueous electrolyte is divided into afirst fluid-contacting the first metal and a second fluid contacting thesecond metal said first and second fluids being coupled by a selectivelypermeable device hindering passage of ions of the first metal to thesecond fluid.

This measure results in a remarkable increase of the dissolution rate.

The invention is advantageously used in removing a coating from a metalsubstrate, e.g. Zn or Sn from metal scrap, preferably steel scrap.

According to the invention a known apparatus having first and secondprocess volumes coupled by a device capable of hindering the passage ofions can advantageously be used for galvanic dissolution of Zn or Sn.

The invention is also embodied in a method for treating Zn-containingsteel scrap by electrochemically dezincing in an alkaline solution in afirst process and reclaiming the zinc in a second process characterizedin that the dezincing in the first process takes place galvanically,i.e. without external electrical power supply. In such a methodconsiderable savings are achieved in that the dezincing takes placewithout external electrical power supply.

It is believed that in a galvanic process, inhibition of the hydrogenevolution reaction (HER) is caused by the occurrence of a phenomenonthat may be called under potential deposition (UPD), which means that ina method according to the preamble of claim 1, although the first metalwill not form a massive deposit on the second metal, it tends to form a(sub-)monolayer at the surface of the second metal, which apparentlyhinders evolution of hydrogen.

Since the dissolution rate and the hydrogen evolution correspond, byreducing the inhibition of the hydrogen evolution at the second metal,according to the invention the dissolution of the first metal can bepromoted.

BRIEF DESCRIPTION OF DRAWINGS

Reference is made to the drawings wherein:

FIG. 1 represents the dissolution rate of a first metal, both in casethe first metal is isolated and in case it is coupled to a second metal,which has a larger exchange current density for the HER than the firstmetal;

FIG. 2 represents the dissolution rate of Zn intrinsically coupled tosteel as a function of Zn dissolved (7.5M NaOH, 298 K);

FIG. 3 represents the dissolution rate of Zn, both isolated andintrinsically coupled to steel, as a function of NaOH concentration (2 gl⁻¹ Zn dissolved, 343 K);

FIG. 4 represents the dissolution rate of Zn intrinsically coupled tosteel as a function of NaOH concentration (2 g l⁻¹ Zn dissolved, 298 K);

FIG. 5 represents the dissolution rate of Zn intrinsically coupled tosteel as a function of NaOH concentration (2 g l⁻¹ Zn dissolved, 323 and343 K);

FIG. 6 represents the dissolution rate of Zn intrinsically coupled tosteel as a function of temperature (7.5M NaOH, 10 g l⁻¹ Zn dissolved);

FIG. 7 represents the dissolution rate of Zn intrinsically coupled tosteel as a function of temperature (2.5M NaOH, 2 g l⁻¹ Zn dissolved);

FIG. 8 represents the dissolution rate of Zn intrinsically coupled tosteel as a function of temperature (7.5M NaOH, 2 g l⁻¹ Zn dissolved);

FIG. 9 represents linear sweep voltammograms (scan rate 1 mV s⁻¹) of asteel electrode (7.5M NaOH, 298 K, various amounts of Zn dissolved asindicated);

FIG. 10 represents the inhibition factor as a function of Zn dissolved(7.5M NaOH, 298 K);

FIG. 11 represents a cyclic voltammogram (scan rate 1 mVs⁻¹) of a steelelectrode (7.5M NaOH, 3 g l⁻¹ dissolved, 343 K);

FIG. 12 represents a schematic presentation of a coupled currentexperiment without a barrier between the anodic and cathodiccompartment; the resistor in the external circuit was varied asindicated; anodic compartment (7.5M NaOH, 5 g l⁻¹ Zn dissolved, 298 K);cathodic compartment same as anodic compartment;

FIG. 13 represents the anodic current at E_(zn) and the cathodic currentat E_(Fe) in case of the experimental set-up as depicted in FIG. 12;

FIG. 14 represents a schematic presentation of a coupled currentexperiment with a barrier between the anodic and cathodic compartment;the resistor in the external circuit was varied as indicated; anodiccompartment (7.5M NaOH, 5 g l⁻¹ Zn dissolved, 298 K); cathodiccompartment (7.5M NaOH, 298 K);

FIG. 15 represents the anodic current at E_(zn) and the cathodic currentat E_(Fe) in case of the experimental set-up as depicted in FIG. 14;

FIG. 16 represents the cathodic current as a function of E_(Fe) for thecoupled current experiments without and with a barrier between theanodic an cathodic compartmsent;

FIG. 17 same as FIG. 16, but now the cathodic current as a function ofE_(Fe) of the coupled current experiment with a barrier between theanodic and cathodic compartment has been extrapolated to tore negativepotentials making use of the Butler-VoLmer equation;

FIG. 18 represents the affect of scratching the surface of a steelelectrode on the HER (7.5M NaOH, 3 g l⁻¹ dissolved, 343 K;

Wherein all potentials referred to are measured against a Ag/AgCl,KCl(saturated) reference electrode, which has a potential of 0.197 V vs.NHE (Normal Hydrogen Electrode).

The invention will now be demonstrated by way of non-limitative examplescomprising results of experiments.

For spontaneous dissolution of a first metal, being denoted M₁ forconvenience, in an aqueous electrolyte to occur, some requirements willhave to be fulfilled, which are outlined below.

First of all, the first metal M₁ should act as an anode:

    M.sub.1 →M.sub.1.sup.a+ +ne

wherein n is the number of electrons per atom oxidized. Electrons, beingreleased as a result of the anodic reaction, are readily consumed in acorresponding cathodic reaction. The cathodic reaction in the underlyinginvention is the hydrogen evolution reaction (HER):

    nH.sup.+ +ne→1/2nH.sub.2 (g) (acid solution)

    nH.sub.2 O+ne→1/2nH.sub.2 (g)+nOH.sup.-  (alkaline solution)

So, both reactions proceed hand in hand. In case of spontaneousdissolution, the reversible (open circuit) cell potential E_(cell),which is defined as the cathodic potential E_(c) minus the anodicpotential E_(a), should be positive. This case is generally beingreferred to as a galvanic cell in contrast to an electrolytic cell. Incase of an electrolytic cell E_(cell) <0, a potential difference has tobe applied, with the help of an external device, like a rectifier, toenforce the electrode reactions to proceed in a direction opposite totheir spontaneous tendencies. The invention is related to galvaniccells, so E_(cell) >0. This condition holds for both Zn and Sn and moregenerally for all metals ranking negative in the electrochemical series,in both an acid and an alkaline environment. These metals willspontaneously dissolve, whereby simultaneously the HER will take placeat their surface. However, the latter reaction proceeds very slowly atboth a Zn and a Sn surface. Consequently, the HER determines the rate ofthe overall reaction. In electrochemical terms the rate of a particularelectrode reaction is being expressed by its exchange current density(symbol:i₀). A `slow` electrode is being characterized by a low i₀ (H₂O→H₂). The HER at a Zn surface has an exchange current density in theorder of 10⁻¹⁰.5 A cm⁻². The dissolution rate of M₁ can be increasedsignrificantly by galvanic coupling of M₁ a second metal M₂, having alarger exchange current density for the HER than M₁, like Pt, Pd, Ir,Co, Ni and Fe or steel, in the case of M₁ being Zn or Sn. In case of Fe,i₀ (H₂ O→H₂)≈10⁻⁵.5 A cm⁻².

The effect of galvanic coupling of M₁ to a foreign metallic substrate M₂is depicted in FIG. 1. If M₁ is immersed into an aqueous solution, itwill adopt a mixed potential, called the corrosion potential, at whichthe anodic current equals the cathodic current, which current is calledthe corrosion current, but, because corrosion implies an unwanteddeterioration of a metal, here this current will be referred to as thedissolution current. In the case of M₁ coupled to M₂, the mixedpotential is shifted in a positive direction, resulting in a largerdissolution current.

In order to study the effect of galvanic coupling on the dissolutionrate, experiments were carried out on one-sided galvanized steel, inwhich case galvanic coupling is intrinsic. As a reference material, alsotwo-sided galvanized steel and pure Zn have been used in someexperiments. Coupons of 6.5×5.5 cm² were prepared and exposed to NaOHsolutions. The dissolution rate was determined by weight-lossexperiments. The time of exposure, amount of Zn dissolved, NaOHconcentration and temperature were varied. All experiments were carriedout at least in duplo. The spread in numerical results was marginal.

EXAMPLE 1

Coupons of one-sided galvanized steel were exposed to a 7.5M NaOHsolution at 298 K with a different amount of Zn dissolved. As is seen inFIG. 2, the dissolution rate of Zn is slowed down drastically once asmall amount of Zn is dissolved. This effect is very much larger thanexpected from calculations. These calculations revealed that this effectcan not be explained by assuming Butler-Volmer kinetics, correcting forthe shift in the reversible potential of the Zn(OH)₄ ²⁻ /Zn redoxcouple, which was commuted with the Nernst equation.

EXAMPLE 2

In a further experiment coupons of one-sided and two-sided galvanizedsteel, as well as coupons of pure Zn, were exposed to NaOH solutions ofdifferent alkalinity. From FIG. 3, by comparing the experiments ontwo-sided galvanized steel, i.e. without coupling to steel, with theexperiments on one-sided galvanized steel, i.e. coupling to steel beingintrinsic, it is seen that the dissolution rate is increasedconsiderably by galvanic coupling to steel. Also experiments werecarried out on pure Zn, which results were similar to results obtainedfrom experiments carried out on two-sided galvanized steel. Noteworthyis the inflection point at higher NaOH concentrations. Where thedissolution rate of Zn only slowly increases up to a concentration ofabout 8M, it suddenly unexpectedly increases sharply at higherconcentrations. In case of two-sided galvanized steel, the dissolutionrate is almost independent of the NaOH concentration. The remarkableincrease in the dissolution rate or Zn at higher NaOH concentrations hasbeen invariably reproduced under various experimental conditions (seeFIGS. 4-5).

EXAMPLE 3

In a further experiment coupons of one-sided galvanized steel wereexposed to NaOH solutions at different temperatures. From FIGS. 6-8 itis seen that the dissolution rate of Zn increases sharply at highertemperatures. Surprisingly, Arrhenius plots, ln(dissolution rate) vs.(1/T), gave no straight lines, but indicated that at higher temperaturesthe increase in the dissolution rate is very much larger than expected.

EXAMPLE 4

In view of the experiments described above, the HER at a steel surfacewas studied in more detail, as this reaction, as already mentionedbefore, determines the dissolution rate. A steel electrode was immersedinto NaOH solutions, together with a Pt counter electrode and a Ag/AgCl,KCl(saturated) reference electrode. using a potentiostat, it waspossible to control the potential of the steel electrode with respect tothe reference electrode. The potential of the steel electrode was variedlinearly with time with a scan rate of 1 mV s⁻¹ in the negativedirection and simultaneously the current was measured. In FIG. 9. linearsweep voltammograms are plotted at various amounts of Zn dissolved.Clearly, the presence of only a small amount of Zn dissolved, alreadyslows down the HER at a steel surface drastically. Apparently a form ofunderpctential deposition (UPD) of Zn on steel occurs, whereby a(sub-)monolayer of Zn is deposited on steel. The extent to which the HERis inhibited has been evaluated experimentally at various concentrationsof Zn dissolved (FIG. 10). 5 g l⁻¹ Zn dissolved inhibits the dissolutionrate by a factor 150, at 298 K. Also, the coverage of the steel surfacehas been evaluated. coverage here means the extent to which the activesurface of the second metal is covered with ions of the first metal. Alogarithmic relationship was found between the Zn(OH)₄ ²⁻ concentrationand the coverage, which indicates that UPD of Zn on steel follows theTemkin adsorption isotherm.

EXAMPLE 5

In a further experiment, similar to example 4, but now the potential ofthe steel electrode was varied linearly with time (scan rate 1 mV s⁻¹),first in the negative direction, then backwards in the positivedirection, and simultaneously the current was measured. FIG. 11 presentsa so-called cyclic voltarmmogram in of a steel electrode in a solutionof 7.5M NaCH with 3 g l⁻¹ Zn dissolved at a temperature of 343 K. Thehysteresis between the forward and the backward scan proves that UPDtakes some tine to develop completely. This is also confirmed bymultiple potential step experiments, wherein the potential was suddenlyswitched from the reversible potential to a potential inside the UPDregion, which invariably show that some time is needed for the currentto become stationary. This opens the opportunity to diminish the effectof UPD by breaking the contact between a galvanic couple of Fe--Zn,before the (sub-)monolayer of Zn has had the chance to developcompletely, Once the contact is broken, both metals will adopt to theirreversible potential. So, the (sub-)monolayer of Zn will dissolve again.Then, the contact is restored, and so on.

EXAMPLE 6

In another experiment, coupled current measurements were carried outwithout and with a barrier, hindering transfer of Zn(OH)₄ ²⁻ ions fromthe anodic to the cathodic compartment, but enabling passage of otherions than Zn(OH)₄ ²⁻, in order to limit ohmic drop over the barrier asmuch as possible.

Coupled current measurements without a barrier:

A Zn bar was immersed in the anodic compartment of a H-cell and a Fe barwas immersed in the cathodic compartment (see FIG. 13). The H-cell wasfilled with an aqueous electrolyte (2.5M NaOH, 5 g l⁻¹ Zn dissolved, 298K). The bars were partly taped with electroplating tape, 3M® No. 484 inorder to expose a well-defined area of 2 cm² to the solution. Betweenthe bars a variable resistor was inserted, over which the potentialdifference was measured with a high input impedance multimeter. In bothcompartments a reference electrode was placed, so that the electrodepotentials could be separately measured. The resistance was graduallyreduced from R=∞ Ω (open circuit) to R=0 Ω (short circuit). The cellcurrent was calculated from Ohm's law. It is seen from FIG. 13 that UPDof Zn manifests itself straightforwardly. Lowering the resistance, thepotential of the Fe bar shifts from its reversible value (open circuit)to its mixed potential (short circuit). It is readily seen that the HERis inhibited drastically, inhibition being strongest at the mixedpotential. Once the cell current has passed its maximum value and isdecreasing, the Zn bar is being less polarised; its potential shiftsback towards its reversible potential. Surprisingly, the maximum currenthas been reached for R=6 Ω. This Ohm's resistance should not beconsidered as the absolute value for which the maximum current isreached in all cases. The important conclusion is that maximum currentis not necessarily achieved at minimum Ohm's resistance. rn other words,there are cases where the cell current may be increased by increasingthe resistance in the circuit. In practice this means that the cellcurrent may be maximized, by increasing, starting from the short circuitsituation, the external resistance until the condition dI/dR=0 has beensatisfied. Once dI/dR=0, E_(Fe) will have attained a value, which willdepend on the Zn(OH)₄ ²⁻ concentration as can be seen from FIG. 9, andE_(zn) will have attained its most positive value.

Coupled current measurements with a barrier:

A barrier was inserted between both compartments (see FIG. 14). Thecompartments were filled with the same solution as in the previousexperiment, but now the cathodic compartment did not contain any Zndissolved. The potential difference between both reference electrodesrepresents the voltage drop over the barrier. From FIG. 15 it is seenthat inhibition of the HER does not occur anymore, leading to largercell currents. In the short circuit situation E_(zn) is not equalanymore to E_(Fe), which is caused by the voltage drop over the barrier,which had a resistance of about 12 Ω. The HER now satisfies theButler-Volmer equation i=10⁻⁵ exp(-17.8(E-E_(eq))) A cm⁻², whereinE_(eq) is the equilibrium potential, which is in good agreement withvalues from literature. By reducing the voltage drop over the barrierthe cell current can be made much larger, because both anodic andcathodic current depend exponentially on potential. Suitable membranesare commercially available, e.g. a Nafion® membrane may be used. In FIG.16, only the cathodic current is plotted vs. E_(Fe), to compare bothcoupled current experiments. In FIG. 17, the cathodic current of thecoupled current experiment with a barrier between the anodic andcathodic compartment has been extrapolated to more negative potentialsof E_(Fe), from which it becomes clear that larger currents will bereached as the voltage drop over the barrier is reduced.

EXAMPLE 7

In a further experiment, the influence of a mechanical treatment of thesecond metal was studied. It is seen from FIG. 18 that scratching of asteel surface has a strong effect on the HER. This effect probablyresults from an increased activity of the surface with respect to theHER. Increasing the activity of the surface is advantageously combinedwith any of the other measures to promote dissolution, since asynergetic effect occurs under all circumstances.

EXAMPLE 8

In another experiment, fine iron powder was added to a beaker of 1 lcontaining a 2:5M NaOH solution with 5 g l⁻¹ Zn dissolved at 353 K. Thesolution was being stirred continuously. Six coupons of two-sidedgalvanized steel were exposed to the solution. The thickness of the Znlayer was 8 μm. Every 5 minutes one coupon was taken out and theeffectiveness of dezincing was being evaluated. It appeared that theamount of iron powder had a strong affect on the dissolution rate of Zn.The time needed for complete dezincing was reduced from 24 minutes at anamount 50 g l⁻¹ iron powder to less than 5 minutes at an amount of 200 gl⁻¹, Adding more powder than 200 g l⁻¹ gave no further improvement.

EXAMPLE 9

In a further experiment it was observed that also Sn coupled to steel orPt leads to acceptable dissolution rates. Because Sn is more noble thanZn (E_(Sn) >E_(Zn)), the dissolution rate of Sn coupled to M₂ wassmaller than in the case of Zn coupled to M₂.

EXAMPLE 10

It was observed that if Mg is used as a current collector no HER occursat the current collector's surface. The specific electrical resistanceof Mg is somewhat larger than that of Cu, but still small enough toconduct considerable currents with negligible ohmic losses. thesequalities make Mg an ideal current collector, if no hydrogen evolutionis desired to occur at the current collector.

As follows from the above examples and experiments several measures canbe taken to obtain favourable dissolution rates of metals such as Sn andZn. As is shown steel scrap can be very economically dezinced ordetinned by processing the scrap in a reservoir made of steel comprisingtwo compartments separated by a membrane in order to prevent UPD in thecathodic compartment. Another possibility to largely prevent UPD is toadd a metal powder of M₂, e.g. iron powder to the reservoir and stir theelectrolyte. By mechanisms as described above, see EXAMPLE 5, very highdissolution rates are obtained. In this case the iron powder may be heldin a confined part of the reservoir by a suitable member in the form ofe.g. a separating screen. Generally, the cathodic/anodic surface rateshould be chosen to be as large as possible for high dissolution rates.

We claim:
 1. A method for electrochemically dissolving a first metal bysimultaneously creating hydrogen evolution at a second metal, the secondmetal being a metal that has a larger current exchange density forhydrogen evolution than the first metal, both metals being immersed inan aqueous electrolyte system, wherein the first metal and the secondmetal are galvanically coupled, comprising applying for reducing theeffect of or preventing under potential deposition of the first metal onthe second metal in order to reduce inhibition of said hydrogenevolution at said second metal, at least one measure of the group ofmeasures consisting of mechanical abrasion of the surface of the secondmetal, adding powder of a second metal to the aqueous electrolytesurrounding the second metal and agitating the aqueous electrolytecontaining the powder, providing an electrical resistance which isselected such that the current flow through connecting meansgalvanically coupling the first and the second metal is substantially atthe maximum value obtainable by varying the resistance periodicallyinterrupting the electrical circuit comprising the connecting means,dividing the aqueous electrolyte into a first fluid for dissolving thefirst metal and a second fluid contacting the second metal said firstand second fluids being coupled by a selectively permeable devicehindering passage of ions of the first metal to the second fluid.
 2. Themethod according to claim 1, wherein the first metal is substantially Znor Sn.
 3. The method according to claim 2, wherein the second metal ischosen from the group comprising Pt, Pd, Ir, Co, Ni, Fe and ferrousmaterials including steel.
 4. The method according to claim 1, whereinthe first metal is Zn.
 5. The method according to claim 4, wherein thesecond metal is Fe or steel.
 6. A method for treating Zn-containingsteel scrap by electrochemically dezincing in an alkaline solution in afirst process and reclaiming the zinc in a second process, wherein thedezincing in the first process takes place galvanically without externalelectrical power supply, according to the method of claim
 4. 7. Themethod according to claim 1, wherein the first metal is Sn.
 8. Themethod according to claim 7, wherein the second metal is Pt.
 9. Themethod according to claim 1, wherein the aqueous electrolyte is one ormore alkaline solution(s).
 10. The method according to claim 9, whereineach said alkaline solution is a sodium hydroxide solution.
 11. Themethod according to claim 10, wherein each alkaline solution is chosento have a hydroxide concentration of more than 9M.
 12. The methodaccording to claim 9, wherein said at least one measure comprises thateach alkaline solution is chosen to have a hydroxide concentration ofmore than 8M.
 13. The method according to claim 9, wherein at least thealkaline solution contacting said second metal is held at a temperatureof above 340 K.
 14. The method according to claim 13, wherein thetemperature of the alkaline solution is above 350 K.
 15. The methodaccording to claim 9, wherein the first metal is in the form of separateelements and the first and the second metal are coupled via a currentcollector contacting the first metal.
 16. The method according to claim15, wherein the current collector has an active surface of Mg.
 17. Themethod according to claim 1, wherein said at least one measure comprisesmechanical abrasion of the surface of the second metal.
 18. The methodaccording to claim 1, wherein said at least one measure comprises addingpowder of a second metal to the aqueous electrolyte surrounding thesecond metal and agitating the aqueous electrolyte containing thepowder.
 19. The method according to claim 1, wherein said at least onemeasure comprises that the first and the second metal are galvanicallycoupled by connecting means, said connecting means providing anelectrical resistance which is selected such that the current flowthrough said connecting means is substantially at the maximum valueobtainable by varying the resistance.
 20. The method according to claim19, wherein the electrical circuit comprising the connecting means isperiodically interrupted.
 21. The method according to claim 1, whereinsaid at least one measure comprises to divide the aqueous electrolyteinto a first fluid for dissolving the first metal and a second fluidcontacting the second metal, said first and second fluids being coupledby a selectively permeable device hindering passage of ions of the firstmetal to the second fluid.
 22. The method according to claim 21, whereinsaid first fluid is in a first process volume for electrochemicallydissolving Zn or Sn and said second fluid is in a second process volumefor hydrogen evolutions, with the first process volume and the secondprocess volume coupled by said device hindering the passage of ions ofthe metal being dissolved, for galvanic dissolution of said Zn or Sn.23. The method according to claim 1, wherein said first metal is in theform of a coating on a metal substrate, and said second metal isseparate from said metal substrate.
 24. The method according to claim 1,comprising removing by said electrochemical dissolution at least onemember of the group consisting of Zn and Sn from metal scrap.
 25. Themethod according to claim 24, comprising removing Zn from steel scrap.26. The method according to claim 24, comprising removing Sn from steelscrap.